The invention relates to a method for the persilylation of carboxylic acid amides of the general formula R--CONH.sub.2 (I) by means of trimethylchlorosilane and tertiary amines, to form the N.N-bis- and/or N.O-bis-trimethylsilylcarboxylic acid amides of the formulas ##STR1## the N.O isomers of formula III being preferred. II was not separated from III.
In these formulas R preferably represents H, CH.sub.3, C.sub.2 H.sub.5, C.sub.3 H.sub.7, CH(CH.sub.3).sub.2, CH.sub.2 .dbd.CH, CH.sub.2 .dbd.C(CH.sub.3), CH.sub.2 F, CHF.sub.2 or CF.sub.3.
Compounds of formula II (R.dbd.CH.sub.3) are formed, according to Chem. Abstr. 57, 11224a, from hexamethyldisilazane by acetylation with ketene, but only in addition to large percentages of the byproducts acetonitrile and hexamethyldisiloxane which are not desirable in this case.
Also the acetylation of hexamethyldisilazane with acetyl chloride gives only moderate yields of products II and III, because the product that has formed reacts with acid chloride, the same as the starting silane, and forms a series of by-products.
The conventional preparation of the substances of formulas II and/or III from amides with trimethylchlorosilane in the presence of tertiary amines as acid acceptors leads to the formation of two equivalents of the tertiary amine hydrochloride in solid form for each amide function of the acid amide. On account of the great amount of solid produced, preparation in the presence of solvents has been attempted (Chem. Abstr. 65, 7025d), but this impairs the completeness of the reaction and hence the yield. The use of trimethylchlorosilane in excess (Chem. Abs. 59, 1673 b) makes the working up of the product difficult and leads to very high chloride contents in the end product. In addition, the danger of delayed reaction exists in this case. Generally, a rather unstable product forms, which tends to degrade to nitriles and hexamethyldisiloxane. This property of the product is especially undesirable on account of poor storage life. Moreover, the yield is still in need of improvement. It has therefore been proposed that amides be reacted with hexamethyldisilazane (G. Schirawski, U. Wannagat, Monatshefte Chem. 1969, 100 (6), 1901-9). In that case, however, monosilylation products form and not products II and III. By-products having a triazine structure are a special disadvantage in this case. Moreover, undesired, unusable ammonia is released when such a procedure is employed.
Accordingly, the problem existed of finding an improved method of preparation without the above-described disadvantages, which above all would deliver a more stable product of good shelf life and improve the yield.